1. Field of the Invention
The present invention relates to a base-station resonator, a filter, a duplexer, and a communication apparatus of a communication facility for use in a microwave band or a millimeter wave band.
2. Description of the Related Art
A conventional resonator is described with reference to FIGS. 6 and 7. FIG. 6 is an exploded perspective view of the conventional resonator, and FIG. 7 is a cross-sectional view taken along line Yxe2x80x94Y of FIG. 6.
As shown in FIGS. 6 and 7, a conventional resonator 110 includes a casing 111 having a main unit 111a and a cover unit 111b and serving as a resonant space, a columnar inner conductor 112 having one end thereof connected to the center of the inner bottom of the casing main unit 111a, screw 113 inserted into the casing 111 from the casing cover unit 111b, and a nut 114 serving as a fixing member for fixing the screw 113. Here, the screw 113 is used to change the resonant frequency of the resonator 110 by varying its insertion position to change the capacitance between the screw 113 and the end of the inner conductor 112.
Generally, since this sort of base-station resonator 110 is used at a high power level, a large amount of heat is generated in use. Accordingly, the casing 111, the inner conductor 112, and the screw 113 are formed from Invar or the like, which is expensive but has very small linear expansivity (coefficient of thermal expansion), so that characteristics such as the resonant frequency are not changed due to a change in the position of the screw 113 or the change in the resonant space itself specified by the size or shape of the casing 111 that is caused by a change in the temperature. Alternatively, the casing 111 is formed from an inexpensive material such as aluminum, and the inner conductor 112 and the screw 113 are formed from a different material whose linear expansivity is different from that of the casing. In this case, the material of the inner conductor 112 and the screw 113 is selected from suitable materials so as to offset any change in the resonant frequency due to a change in the resonant space constituted by the casing 111 that is caused by a short-term temperature change. On the other hand, the nut 114 for fixing the screw 113 is formed using an inexpensive metal such as commonly distributed iron.
As described above, the casing and the screw of the conventional resonator are made from a metal such as Invar and the nut for fixing the screw is made from a metal such as iron. Accordingly, when a long-term cyclical temperature change is caused by heat generated from the resonator itself or a change in the external environment in use, a difference develops between the force the casing applies to the screw and the force the nut applies to the screw, due to the difference between the linear expansivity of the casing and the linear expansivity of the nut, or the difference between that of the screw and that of the nut. This force difference loosens the screw. Since this changes the position of the screw, there arises a problem that the resonant frequency deteriorates with age.
When the resonator is manufactured, an aging process is required, normally at a high temperature, in order to ensure the reliability of the resonator characteristics. However, even though the screw insertion position is adjusted and then fixed with the nut, the above linear expansivity difference causes the resonant frequency of the resonator to be changed during the aging process. Therefore, there is a problem that readjustment is required.
In order to address these problems, the present invention provides a resonator, a filter, a duplexer, and a communication apparatus wherein characteristics such as the resonant frequency are stable for a long period regardless of the occurrence of cyclical changes in the temperature.
To this end, according to a first aspect of the present invention, there is provided a resonator having a conductive casing provided with a threaded screw hole, a screw at least partially inserted into the screw hole, and a fixing member fixing the screw and also having a threaded screw hole. In the resonator, the linear expansivity of the casing and the linear expansivity of the fixing member are substantially equal.
When the linear expansivities of the casing and the fixing member are substantially the same in this manner, the looseness of the screw due to the cyclic temperature change does not occur. This causes the position of the screw to be stabilized. Therefore, the stability of the resonant frequency with age is increased.
In the resonator, the linear expansivity of the casing, the linear expansivity of the fixing member, and the linear expansivity of the screw may be substantially equal. By causing not only the linear expansivities of the casing and the fixing member to be substantially equal, but also causing the linear expansivities of the casing and the fixing member to be substantially equal to that of the screw, the stability of the resonant frequency with age is increased even more.
According to a second aspect of the present invention, a resonator includes a conductive casing provided with a threaded screw hole, a screw at least partially inserted into the casing, and a fixing member fixing the screw and having a threaded screw hole. In the resonator, the casing and the fixing member are made from the same material.
Thus, when the casing and the fixing member constituting the resonator are each formed using a metal such as Invar, since there is no difference in the linear expansivities of the casing and the screw, the screw is not loosened. Therefore the stability of the resonant frequency with age is increased.
In the resonator, the casing, the fixing member, and the screw may be made from the same material. When not only the casing and the fixing member but also the screw are all formed using a metal such as Invar, the stability of the resonant frequency with age is increased even more.
Alternatively, in the resonator, one end of a rod-shaped inner conductor is connected to the inner wall face of the casing, and the axis of the inner conductor and the axis of the screw are substantially aligned.
In the resonator having the casing provided with the inner conductor in which one end thereof is connected and the other end thereof is open, the electrical field intensity of the open end is great. Accordingly, when the axis of the inner conductor and the axis of the screw are substantially aligned, as the capacitance between the open end of the inner conductor and the screw increases, the proportion of the amount of the change in the capacitance to the amount of the change in the screw position increases. This means that, in the resonator having such a construction, the resonant frequency is sensitively changed in accordance with the position of the screw. Particularly, a characteristic change due to mechanical aging caused by long-term cyclic temperature change is a great problem in the resonator having such a construction. The present invention is very effective particularly in the resonator having such a construction.
Furthermore, in the resonator, the number of threads of the casing that engage with the screw and the number of threads of the fixing member that engage with the screw may be substantially equal. When the screw is inserted into the casing and the screw is fixed with the fixing member, the screw is pulled downward by the casing and pulled upward by the fixing member. These pulling forces are balanced. When a temperature change subtly alters the forces applied by the casing and the fixing member to the screw, the one having more threads can have more influence on the screw. That is, by causing the number of threads of the casing and the number of threads of the fixing member to be substantially equal, a resonator which is stable over a longer-term cyclic temperature change can be obtained.
According to a third aspect of the present invention, a filter includes the above-described resonator, and an input/output connection unit.
According to a fourth aspect of the present invention, a duplexer includes at least two filters, input/output connection units connected respectfully to each filter, and an antenna connection unit commonly connected to the filters. In the duplexer, at least one of the filters is the above-described filter.
According to a fifth aspect of the present invention, a communication apparatus includes the above-described duplexer, a transmission circuit connected to at least one of the input/output connection units of the duplexer, a reception circuit connected to at least one of the input/output connection units that is different from the input/output connection unit connected to the transmission circuit, and an antenna connected to the antenna connection unit of the duplexer.
The communication apparatus may further comprise at least one of a transmitting circuit and a receiving circuit, and the above-described filter being connected to or forming a part of said circuit.
As described above, according to the present invention, in the resonator which includes the casing, the inner conductor provided in the casing, the screw partly inserted into the casing, and the fixing member for fixing the screw, the difference between the linear expansivity of the casing and the linear expansivity of the fixing member, or the difference between that of the casing or the screw and that of the fixing member, is substantially eliminated. This prevents the resonant frequency of the resonator from being varied due to the looseness of the screw that is caused by the long-term cyclic temperature change in use or the temperature change during an aging process. Accordingly, the long-term reliability of the resonator is improved.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.